The present invention relates to recirculating pumps for nuclear reactors in general, and more particularly to improvements in internal primary pumps for recirculation of coolant in pressure vessels of boiling water reactors.
It is already known to provide in a nuclear reactor plant a battery of internal recirculating pumps which extend into the bottom wall of the pressure vessel in a boiling water reactor. Such recirculating pumps are installed independently of the feed water supply lines and are driven by normal electric motors or by so-called wet or canned motors known from the art of glandless recirculating pumps. A drawback of such constructions is that the pressure vessel of the reactor must be provided with several sets of openings, namely, those for the introduction of portions of internal recirculating pumps and those for the admission of feed water. Moreover, conventional glandless recirculating pumps constitute expensive auxiliary aggregates especially since, for the reasons of safety, each such pump normally embodies a motor generator with flywheel for supplying additional energy in order to prolong the period of deceleration of the pump rotor in the event of current failure. Gradual deceleration is desirable and necessary in order to insure satisfactory cooling of the pressure vessel. In the absence of aforementioned generators, the rotor of the pump would be arrested practically immediately upon opening of the motor circuit.
It was already proposed to employ in a boiling water reactor one or more internal recirculating pumps which are driven by water turbines and are mounted in such a way that feed water enters the pressure vessel by way of openings which are provided for introduction of portions of internal recirculating pumps. Reference may be had to German Offenlegungsschrift No. 1,921,903. This renders it possible to reduce the number of openings in the pressure vessel and the number of conduits for admission of feed water. However, such pressure vessels must be equipped with bypass lines. Another drawback of the just described recirculating systems is that the regulation of water turbines is extremely complex, mainly because the blades of the turbine are not readily accessible for adjustment of their angles.